This invention relates generally to an energy conversion system and, more specifically, to an energy conversion system which is capable of converting three-phase high frequency energy to single phase, relatively low frequency energy as, for example, 60 cycle energy.
With the increased use of electrical energy in remote areas, a great demand has arisen for portable or emergency energy sources of electric power. Typically, these sources of electrical power are utilized to drive appliances and the like which are designed to efficiently operate at 60 hertz alternating power. Accordingly, there is a need for a portable source of alternating current, 60 hertz energy which is adapted to utilize the highly developed components readily available on the market.
As is well known, there is no readily available practical alternator system, in the conventional sense, with less than two poles. This limitation results in a maximum shaft speed of the 60 hertz alternating current generator of 3,600 rpm. This relatively slow shaft speed necessitates the use of heavy iron structures throughout the machine with subsequent high cost and lower overall efficiency. Further, there have become available highly developed alternators, for example, the alternators utilized in automotive use, and high horsepower-to-weight ratio prime, i.e., high shaft speed, movers which, with the features of the present invention, may be utilized to generage 60 cycle alternating current.
As stated above, the most common means of generating alternating current energy is by means of a generator driven by a prime mover. It is known that the size, and therefore the expense, of building a generator to produce a preselected power output decreases as the frequency increases. However, since the standard in the industry has become 60 hertz in the United States, and in many other countries of the world, the use of high frequency generators has not been practical for many applications.
This situation has been overcome with the system of the present invention which provides a means of utilizing a high frequency generator incorporating highly developed prime mover and generator components presently available on the market to produce a low frequency output.
This is accomplished by utilizing a conventionally available prime mover which is highly efficient, e.g, 7,200 rpm, to drive a high speed, three-phase, multipole, e.g. 12 to 16 poles, alternator of conventional design.
In a preferred form of the invention, the prime mover is operated at 7,200 rpm to rotate the field windings of a three-phase twelve pole alternator which is adapted to generate a 720 hertz output from a polyphase wye connected armature winding. The output of the three-phase armature winding is approximately equal to the desired peak voltage of the sinusoid waveform that the system reconstructs at the output. In the preferred form, this output voltage is approximately 160 volts. For purposes of this specification, the use of the term hertz is intended to include a repetitive waveform, whether alternating or unipolar.
The output of the three-phase 720 hertz alternator is fed to a full wave rectifier bridge, half of the rectifier bridge in one form of the invention being comprised of controlled rectifiers having a control electrode. The main current path of the output of the full wave bridge is fed to an inverter device which is utilized to invert alternate cycles of the output from the full wave bridge. The control electrode of the controlled rectifiers is connected to a resolver device which is utilized to control the firing of the controlled rectifiers in accordance with the synchronous operation of the prime mover. In a second form of the invention a modified resolver is used in place of the controlled rectifiers to control the rectification.
Accordingly, the switching of the power output of the controlled rectifiers or modified resolver is accomplished by the inverter device and occurs twice per output cycle. The inverter is devised and synchronized to perform its switching function at a point when the output voltage is zero. The inverter is not required to switch high currents with resistive loads; however where inductive loads are encountered an arc suppressor circuit can be provided.
As will be seen from a further description of the system, the output from the inverter is not, in the voltage sense, a sinusoidal waveform. Rather, the output is approximately at a constant voltage, for example 160 volts. However, the average output current generated at the output terminals of the inverter varies as a 60 hertz sinusoidal wave. This is accomplished by first switching to the conductive state the first switching device i.e. controlled rectifier or modified resolver to provide a short duration pulse of 160 volt peak amplitude. Subsequently, a second switching device is switched in to add to the output energy through the first switching device. In this situation, the average output current increases approximately by a factor of two from the original output pulses. Subsequently, the third switching device is switched in whereby all three switches and hence all phases are conducting and energy is flowing from the output inverter substantially continuously.
The situation then reverses whereby the first switching device is switched off and the second and third devices conduct to provide approximately two-thirds of the full load average current, and finally the second switching device is switched off leaving only the third switching device. At the end of the cycle, all three devices and hence phases are turned off and the inverter switches the subsequent waveform to the opposite polarity and presents this inverter waveform to the output of the inverter. From the foregoing, it is seen that each phase can provide approximately an equal amount of output current relative to the remaining phases. Thus, no one phase need provide substantially more energy than any other phase in the system. This operation becomes more apparent from a detailed study of FIGURE 2 to be described hereinafter.
Accordingly, it is one object of the present invention to provide an improved prime mover and generator set for generating alternating current low frequency energy.
It is another object of the present invention to provide an improved prime mover and generator set wherein the generator operates at a high frequency and the high frequency energy is converted to a relatively low frequency.
It is another object of the present invention to provide an improved prime mover and generator set wherein the output of the generator is converted to an average current having a waveform substantially sinusoidal in shape.
It is still another object of the present invention to provide an electrical energy conversion system wherein the generator portion of the system generates electrical energy at a relatively high frequency and this high frequency energy is converted to a low frequency, the low frequency being alternating in form and has a power envelope which is approximately equivalent to that of a sinusoidal waveform.
It is still a further object of the present invention to provide an improved energy conversion system which utilizes a multiphase, high frequency generator source for generating single phase, low frequency electrical energy.
It is still another object of the present invention to provide an improved energy conversion system which converts multiphase, high frequency energy to single phase, low frequency energy wherein the output voltage amplitude of the output energy is substantially constant.
It is another object of the present invention to provide an improved energy conversion system which converts multiphase, high frequency alternating current energy to single phase, low frequency alternating current energy wherein the power output of the system is a time modulated waveform having a sinusoidal characteristic.
It is a further object of the present invention to provide an improved energy conversion system which utilizes a high frequency multiphase electrical generator, the output of the generator being fed through controllable devices to control the feeding of the energy from the generator to the output in accordance with a desired average power output waveform.
It is a further object of the present invention to provide an improved electrical energy conversion system which utilizes a multiphase high frequency generator, the output thereof being fed to an output circuit wherein certain switching functions are performed within the system.
It is another object of the present invention to provide an improved prime mover generator set energy conversion system which obviates the use of large ferrous structures within the system while generating relatively low frequency output energy.
It is another object of the present invention to provide an improved prime mover and generator set having an extremely favorable frequency-to-weight power ratio.
It is still a further object of the present invention to provide an improved prime mover and generator set energy conversion system which is capable of utilizing high speed, light weight, automotive type alternators to produce the output energy.
It is still another object of the present invention to provide an improved prime mover and generator set energy conversion system for generating relatively low frequency output energy having a sinusoidal characteristic wherein the prime mover is operable at optimum shaft speeds above 3600 rpm.
It is another object of the present invention to provide an improved excitation circuit for a generator of the type described.
It is still another object of the present invention to provide an improved energy conversion system which is efficient and reliable in operation, inexpensive to manufacture, and highly portable.